Valence (chemistry)

History

The etymology of the word "valence" is from 1425, meaning "extract, preparation," from Latin valentia "strength, capacity," and the chemical meaning referring to the "combining power of an element" is recorded from 1884, from German Valenz.^[1]

In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds.^[2] If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and similarly for the other combinations of ultimate particles:

The exact inception, however, of the theory of chemical valencies can be traced to an 1852 paper by chemical affinity to show that certain elements have the tendency to combine with other elements to form compounds containing 3, i.e. in the three atom groups (e.g. NO₃, NH₃, NI₃, etc.) or 5, i.e. in the five atom groups (e.g. NO₅, NH₄O, PO₅, etc.), equivalents of the attached elements. It is in this manner, according to Franklin, that their affinities are best satisfied. Following these examples and postulates, Franklin declares how obvious it is that:^[3]

“

A tendency or law prevails (here), and that, no matter what the characters of the uniting atoms may be, the combining power of the attracting element, if I may be allowed the term, is always satisfied by the same number of these atoms.

”

This “combining power” was afterwards called quantivalence or valency (and valence by American chemists).^[2]

Overview

The concept was developed in the middle of the nineteenth century in an attempt to rationalize the covalent bonds rather than "valence", which has fallen out of use in higher level work with the advances in the theory of chemical bonding, but is still widely used in elementary studies where it provides a heuristic introduction to the subject.

"Number of bonds" definition

The number of bonds formed by a given element was originally thought to be a fixed chemical property and in fact, in many cases, this is a good approximation. For example, in many of their compounds, oxidation numbers (used in Stock nomenclature) and to lambda notation in the IUPAC nomenclature of inorganic chemistry.

IUPAC definition

The International Union of Pure and Applied Chemistry (IUPAC) has made several attempts to arrive at an unambiguous definition of valence. The current version, adopted in 1994,^[4]:

The maximum number of univalent atoms (originally chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.

This definition reimposes a unique valence for each element at the expense of neglecting, in many cases, a large part of its chemistry.

The mention of hydrogen and chlorine is for historic reasons, although both in practice mostly form compounds in which their atoms form a single bond. Exceptions in the case of hydrogen include the ion [HF₂]⁻ and the various boron hydrides such as Fluorine is the element for which the largest number of atoms combine with atoms of other elements: it is univalent in all compounds except the ion [H₂F]⁺. In fact, the IUPAC definition can only be resolved by fixing the valences of hydrogen and fluorine as one, a convention which has been followed here.

Valences of the elements

Valences for the majority of elements are based on the highest known fluoride.^[5]

Group →	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	1 H																	2 He
2	3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg											13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba	*	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra	**	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Uub	113 Uut	114 Uuq	115 Uup	116 Uuh	117 Uus	118 Uuo
* Lanthanides				57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu
** Actinides				89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr

Valences of chemical elements      None      One      Two      Three      Four      Five      Six      Seven
standard temperature and pressure (0 °C and 1 atm) Solids Liquids Gases	Borders show natural occurrence Primordial From decay Synthetic Undiscovered

Other criticisms of the concept of valence

• The valence of an element is not always equal to its lowest perchlorates).
• The concept of "combination" cannot be equated with the number of bonds formed by an atom. In lithium fluoride (which has the lithium atom is surrounded by six fluorine atoms, whereas the valence of lithium is universally taken to be one, as the formula LiF would suggest.^[6]

See also

• Chemical affinity

References

1. ^ Valence - Online Etymology Dictionary.
2. ^ ^{a b} Partington, J.R. (1989). A Short History of Chemistry. Dover Publications, Inc. ISBN 0-486-65977-1. 
3. ^ Franklin, E. (1852). Phil. Trans., vol. cxlii, 417.
4. ^ Pure Appl. Chem. 66: 1175 (1994).
5. ^ http://www.webelements.com/ (accessed 2006-02-20).
6. ^ In the gas phase, LiF does indeed exist as discrete diatomic molecules as the valences would suggest: Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.